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Sprinkler Valve Question

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Sprinkler Valve Question

Unread postAuthor: Hubb » Sat Aug 23, 2008 8:23 am

I'm sure a camera circuit will trigger a solenoid, but is it too much voltage to fry it?

My guess is no, as the current will not remain long enough, but I'm not electronics expert, either?

Anyone ever tried this or have input?
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Unread postAuthor: jonnyboy » Sat Aug 23, 2008 8:27 am

Camera circuits are about 370 volts aren't they? Sprinkler valves go on about 30 volts so you might get a shot or two but it will fry it. Don't take my word as law I don't know much either but I know electronics don't like a lot of volts.
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Unread postAuthor: clide » Sat Aug 23, 2008 12:52 pm

It should work fine. It's been done before, and I've made a small coil gun out of a sprinkler valve solenoid and a camera flash.

Although there is a question of why? If you are opening a sprinkler valve with the solenoid then the opening speed of the actual solenoid isn't much of a factor. I suppose it would allow you to enlarge the solenoid exhaust hole though, which would be good.
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Unread postAuthor: Hubb » Sat Aug 23, 2008 1:13 pm

Although there is a question of why?


I'm designing a launcher that will be electronically triggered. Once triggered, the switch will remain open (this will clear itself up in due time). If the valve is opened with a capacitor and not a battery, the switch being left open is irrelevant. It will not drain the battery AND one AA battery is a lot cheaper than 3 9volt batteries.
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Unread postAuthor: clide » Sat Aug 23, 2008 1:23 pm

Ah fair enough. I could be wrong on this, but on my camera flash it seems like if I left the switch on after I triggered it it would still drain the battery because it was constantly trying (and failing) to refill the capacitor.
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Unread postAuthor: starman » Sat Aug 23, 2008 1:28 pm

Those 3 - 9V batteries will last longer than you think...the current draw to actuate the solenoid for the shot will only last for a second or so.

A camera circuit to drive a solenoid sounds a little sketchy. It may be able to apparently work, but who knows the safety and coil limit issues you may come up on. You are dealing with what's considered HV there.
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Unread postAuthor: Hubb » Sat Aug 23, 2008 1:41 pm

If the switch is left on, the 3 9volt batteries will draw more power than a bump from a capacitor.

clide: the circuit would be broke, meaning it would draw no power.

I have a solenoid going to waste. I'll see how extreme I can push it with a camera capacitor.
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Unread postAuthor: jimmy101 » Sat Aug 23, 2008 4:05 pm

A 9V battery (or 3 9Vs in series) will source about 1 amp. To get better performance you need to be able to boost that 1 amp current limit.

There's a couple ways to boost the current, you can boost the voltage across the solenoid's coil or you can reduce the reistance of the circuit. The internal reistance of a 9V is probably about 9 Ohms (9V/1A). Three 9Vs in series roughy 30 Ohms. Figure the resistance of the solenoid's coil is about 10 Ohms. If you could reduce the batteries resistance to a couple ohms you could significantly increase the current. One way to reduce the resistance of the 9Vs is to put them in parallel with a capacitor. The internal resistance of a cap's is typically less than a 0.1 Ohms. The cap basically lowers the resistance of the circuit for a short period pulse. So you charge up the cap (it's just permanently wired across the batteries) then dump the cap + battery into the coil via a pushbutton switch.

For the cap to make much of a difference it has to have a pretty high capacitance since it needs to be able to source significantly more than the 1 Amp the battery can. If you stay with a 9V (or 18V or 27V) source the cap needs a very large value, probably in the millifarad range for it to actually help. Millifrad sized caps aren't all that common. The typical photoflash cap is 100~150 microfarads (MFD).

Since a cap probably won't help all that much at 9V, 18V or 27V, boosting the voltage and using the cap is a reasonable approach. 300~320V (typical disposable camera voltages) is kind of overkill but is probably OK. The cap will discharge pretty quickly, the ~70% discharge time (Farads*Ohms) is pretty short, which is good because 300V continous through the coil will fry it for sure. For a 120 MFD cap and a 10 Ohm coil the time to ~70% discharge of the cap would be about 1 millisecond (ignoring the inductance of the solenoid coil which is probably pretty large). For the cap to discharge all the way down to the 24V the soenoid is diesigned for would take about 3mS and the cap would still be sourcing a couple amps. After 3mS the voltage (and current) from the cap will probably be below what is needed to hold the solenoid open. That's the drawback of the high voltage cap, it'll drop to below the voltage and current required to hold the solenoid open pretty quickly. You would have to fiddle with the cap and voltage to get a long enough pulse for your needs.

The battery + cap combination doesn't drop below the solenoids hold voltage and current as the cap discharges, the circuit just drops to whatever the battery(ies) can do all by themselves.

I used Barry's RLC simulator to get the times, voltages and currents assuming a 1mH coil inductance and 10 Ohm coil resistance.


You might also consider porting the valve, that might help more than boosting the opening time of the solenoid. Like clide said, the solenoid may not be the limiting factor in the valve. To port the valve you remove the solenoid and carefully drill out the port hole. If you can increase the diameter of the hole by just 40% you'll increase the flow through the port by nearly 200% (assuming the port itself is the major flow restriction in the port pathway).
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